I must admit to finding this whole thing very confusing, all supermarket branches seem to be doing their own thing, and there is no way of knowing what to do aside from getting scolded and spoken down to by supermarket staff.
Last week we went to the coast and stopped off at a Morrisons in a small south coast town. There was one person at the checkout who was in the process of paying so we walked up to the belt and was told to join a (non existent) queue in one of the aisles. About 3 Minutes later we were back at the same checkout, in the same position.
Yesterday, on another daytrip, we went shopping in another town, outside of London, and the man on the door asked us, when we went in to keep apart. Thinking I'd misheard, I'd asked him to repeat and he said it again. I was totally baffled by this, as in other shops I've been into, they ask where you have two people shopping together that you try and stay together whilst in the shop and not split up!